In this post we’ll head further out into the solar system and look at two of Saturn’s moons, Enceladus and Titan.

Like Jupiter, Saturn has many moons; 62 have been found so far, although more may exist. Most of these are small and irregular in shape, and are most likely captured bodies such as asteroids and cometary nuclei. 13 of the moons are larger than 50km across though, and 7 of these are large enough that their gravity has compressed and moulded them into spheres. One of the most famous of these is Mimas, because it looks just like the Death Star. However the two astrobiologists are interested in are Enceladus and Titan.

We’ll start with Enceladus, as it’s story should be familiar to you if you’ve read the previous post about Europa, Ganymede and Callisto (no worries if you haven’t though, it’ll still make sense).

Enceladus is Saturn’s sixth largest moon, is the 14th moon out from Saturn, and is named after a giant from Greek mythology. Enceladus is pretty small; it’s roughly 505 km across, which is one-seventh the diameter of our Moon. Its thought to be made primarily of rock and water ice, with an inner rocky core, and an outer mantle of ice.

As with Europa, Enceladus is geologically active and thought to have a hot core, partly due to tidal heating (see this post for an explanation) and partly due to the radioactive decay of isotopes (basically an internal nuclear reactor). Evidence for this is seen on the moon’s surface, as Enceladus has relatively smooth, crater-free regions and is extremely reflective, which indicates liquid water must periodically flow on its surface, and it has systems of cracks and ridges, indicating its surface is periodically flexed and broken. However the best proof was seen in 2005 when NASA’s Cassini spacecraft examined Enceladus and captured images of volcanism on the Moon’s surface, making Enceladus one of three outer solar system bodies on which active volcanism has been observed (the other two being Io and Triton).

A Cassini image of volcanic plumes erupting on Enceladus

The volcanism isn’t like that seen on Earth or on Io though, where liquid rock erupts on the surface as magma, and then cools to form new rock, as Enceladus has an outer mantle of ice, rather than silicate rock. On Enceladus cryovolcanism, or ice volcanism, occurs, in which ice melts and erupts as water on the surface, rather than magma, and cools to form new ice ‘rock’. In most respects cryovolcanism is the same as volcanism on Earth, but with liquid water in the place of liquid rock, as Enceladus is so cold ice behaves as rock does on Earth.

Now, if you know anything about Europa you’ll know that this is pretty exciting news, yep, you guessed it, Enceladus is also thought to have a sub-surface ocean, possibly extending over the entire sub-surface of the moon. Life could exist within this warm ocean, particularly if hydrothermal vents are present on its floor, with ecosystems potentially similar to the black-smoker ecosystems on Earth. But who knows? Maybe Enceladus has swimming organisms like fish and jellyfish too! Excitingly, Cassini analysed some of the water vapour erupting from Enceladus and found it contained high levels of salt and even organic compounds, strengthening the case for a large sub-surface ocean and the possibility that it could contain life.

A potential sub-surface ocean in Enceladus (click to enlarge)

Titan though, Saturn’s other astrobiologically important moon is quite different from Enceladus. Titan is massive, its by far the largest of Saturn’s moons; it’s so large that if you combined the masses of all Saturn’s moons together it would count for over 96% of that combined mass. In fact it’s so large that it’s bigger than the planet Mercury. Titan’s thought to be composed of a mixture of roughly half rock and half ice, similar to Ganymede and Callisto, differentiated into a rocky core and icy mantle and crust.

Titan’s size compared to the Earth and the Moon

So apart from its size, what makes Titan so special?

Three things. Firstly Titan is only one of three planetary bodies to have a thick atmosphere, other than Venus and the Earth. Secondly, it’s the only other planetary body other than Earth to have long-lasting bodies of liquid on its surface, and thirdly, because it has a rich system of surface and atmospheric organic chemistry (organic chemistry doesn’t actually mean life, it means carbon-based chemistry that life is made out of).

Titan’s atmosphere is denser than Earth’s, with a surface atmospheric pressure about 1.45 times Earth’s; in fact, Titan’s atmosphere is so thick, and its gravity low enough, that humans could swim through its atmosphere, with adequate protection of course!

Its atmosphere is composed primarily of nitrogen, like ours, but is as much as 98% nitrogen, whereas on Earth our atmosphere is around 78%. The remaining 2% is composed of mainly methane and hydrogen, but with trace amounts of more exotic compounds, most of which are hydrocarbons, large molecules mostly made of hydrogen and carbon, such as ethane and propane. Many of these hydrocarbons can condense to liquids in Titan’s atmosphere and thus Titan’s surface likely has hydrocarbon rain, and has lakes, seas and maybe even oceans of liquid hydrocarbons like methane. Titan must be an extremely strange place, with a surface of primarily water ice, lakes of methane, and a thick swimmable atmosphere.

An image taken by the Huygens probe as it landed on Titan in 200x, you can see a river system carving its way through rock, although the river is likely made of methane and the rock made of water ice

A second Huygens image, this time taken from Titan’s surface, showing cobbles of rocky ice eroded by flowing methane

However, in some respects Titan is thought to be similar to the conditions on the early Earth. For the first 2 billion years of our planet’s existence there was very little free oxygen in its atmosphere. Oxygen is an extremely reactive gas and quickly combines with other compounds, from minerals in rocks, to gasses in the atmosphere. Oxygen only became abundant when photosynthetic life evolved and became widespread on our planet, as photosynthesis continually releases oxygen at a greater rate than it reacts and is gobbled up by rocks, water and atmospheric components. It’s though that this happened around 2.4 billion years ago, a time that geologists call the Great Oxygenation Event.

Before this time though, the Earth’s atmosphere may have been similar to Titan’s, primarily composed of nitrogen and hydrogen containing gasses such as methane and other hydrocarbons. And at some point in these first two billion years life emerged on Earth. No one knows exactly how this happened, but many experiments have shown that simple chemicals, many of them hydrocarbons, can react to form larger organic structures such as amino acids, and that these more complex molecules can react to form early cells. Thus scientists think life emerged on Earth by a process called chemical evolution, in which chemical reactions led to the emergence of the building blocks of life, which continued to react, eventually leading to self-replicating biological structures and eventually to the first cells.

Titan may well be a good representation of the early Earth, and we may find examples of reactions on Titan that represent some of the steps of chemical evolution that happened on Earth. Titan most likely won’t have actual living organisms, as it’s surface is so cold and has very little liquid water, that some of the more advanced steps of chemical evolution may never occur, the organic chemistry of Titan is likely frozen at a point of development before life itself emerges. But examining Titan may be like stepping into our own past and seeing those first steps that led to the emergence of life on Earth.

In fact, pretty much everything I’ve said so far is neatly summarised (and far more expertly) in this quick video below, from BBC Horizon’s Titan, a place like home episode:

Next in the series of posts we’ll return a bit closer to home, to Venus.

True, I believe there’s even some hypotheses that life could emerge within other icy bodies, like Kuiper Belt Objects and even in the Oort Cloud. I’ll have to write something about this, as its a pretty amazing idea